1. Field of the Invention
The invention relates to the field of tuneable ultra-high frequency filters and relates more particularly to a dielectric resonator filter of this type in which the resonators resonate in mode TM010.
2. Description of the Prior Art
Recent work in the field of dielectric materials has led to the use of such materials for forming dielectric resonators, having a sufficiently high dielectric constant with acceptable temperature coefficients. Most of the filters formed with these dielectric resonators use resonators having a cylindrical form, (rather than a parallelepipedic or spherical form), which have a better Q factor, are more readily machinable and have a field of use towards the "low" frequencies of the ultra-high frequency range (less than 10 GHz).
This technique has led to considerable improvements in ultra-high frequency filters not only from the point of view of their size but also from the point of view of their performance and price. In fact, if we compare dielectric resonator filters with the different rod, iris or evanescent mode types of filters used previously, it can be seen that:
the Q factor of dielectric resonator filters is slightly less than that of rod or iris filters and very much higher than that of the evanescent mode filters, PA1 dielectric resonator filters take up less space and are lower in weight; PA1 the cost of dielectric resonator filters is substantially equal to that of evanescent mode filters and very much less than those of the filters of the two other types. PA1 a metal case having a longitudinal axis and comprising a base and a lid having a bottom, opposite the base, and forming a plane, the dimensions of this case defining a so called below cut-off ultra-high frequency wave guide, PA1 a succession of cylindrical dielectric resonators disposed inside the case, aligned along its longitudinal axis, having one of their ends in contact with the metal base and their other ends at an adjustable distance from the bottom of the lid for providing frequency tuning of the filter, PA1 input and output coupling means for applying the ultra-high frequency signal to one end of the case and supplying the signal to the other end of the case, PA1 the electromagnetic field resulting from the signal applied being located in and in the vicinity of the dielectric resonators resonating in the TM010 mode.
Tuneable dielectric resonator ultra-high frequency filters, developed up to present, include however a number of drawbacks; first of all, there exists in these filters parasitic modes which may disturb the frequency response of the filters. Moreover, it is difficult to obtain a band width which is variable over a wide range with a filter of a given size. Finally, to obtain tuneability of the filter over a wide band width, tuning systems are provided for varying the tuning frequency of the resonators. These systems generally comprise dielectric rods associated with the resonators, properly speaking, whose penetration is adjustable. These elements provide tuning but they may lower the Q factor obtained when the resonator is alone. Furthermore, they present machining and adjustment problems. Up to present, these filters always used selected modes TE01.delta. or TM01.delta. which have good unloaded Q factors. However, as pointed out above, these coefficients decrease in the complete system, with its tuning means. Contrary to the generally held idea, these modes are not the only ones which may be used in dielectric resonators for forming filters with acceptable characteristics.